Fish condition as an indicator of stock status: Insights from condition index in a food-limiting environment

Individual performance defines population dynamics. Condition index – a ratio of weight and some function of length – has been louded as an indicator of individual performance and recommended as a tool in fisheries management and conservation. However, insufficient understanding of the correlation between individual-level processes and population-level responses hinders its adoption. To this end, we use composite modelling to link individual's condition, expressed through the condition index, to population-level status. We start by modelling ontogeny of European pilchard (Sardina pilchardus, Clupeidae) as a function of food and constant temperature using Dynamic Energy Budget theory. We then provide a framework to simultaneously track the individual- and population-level statistics by incorporating the dynamic energy budget model into an individual-based model. Lastly, we explore the effects of fishing pressure on the statistics in two constant and food-limited environmental carrying capacity scenarios. Results show that, regardless of the species' environmental carrying capacity, individual condition index will increase with fishing mortality, that is, with reduction of stock size. Same patterns are observed for gilthead seabream (Sparus aurata, Sparidae), a significantly different species. Condition index can, therefore, in food-limited populations, be used to (i) estimate population size relative to carrying capacity and (ii) distinguish overfished from underfished populations. Our findings promote a practical way to operationally incorporate the condition index into fisheries management and marine conservation, thus providing additional use for the commonly collected biometric data. Some real-world applications, however, may require additional research to account for other variables such as fluctuating environmental conditions and individual variability.